The plague –nothing conjures up death quite like that word -after all, the bubonic plague wiped out half of Europe in the 14th century. But there have been others of its ilk –and all probably caused by the Yersinia pestis bacterium. Although the yet-unnamed infectious agent was identified in the 1890ies by the bacteriologist Alexandre Yersin -working at the time in the Pasteur Institute on plague samples from an outbreak in Hong Kong- the name was initially misattributed… Never work for somebody really famous when you discover something important. Personally, I preferred its previous name of Pasteurella pestis because that’s the name I was first taught and I liked the alliteration. But never mind.
The plague has three different presentations, depending upon the organs infected: bubonic plague, from infection of the lymphatic system and localized as buboes (swellings of infected lymph nodes which may become necrotic and turn black in their attempt to defend the body); pneumonic plague –infection of the lungs, presumably from aerosolized droplets from coughing or the like; and the rarest and likely most fatal of the three, septicaemic plague, which is an infection of the blood stream. All are carried by fleas, which are carried by rats, which then carry them to us.
Although we tend to associate the word ‘plague’ with the infamous ‘Black Death’ of European fame -not least because of the shock value of its name, I suspect- there have been several plagues throughout history. The first was originally thought to have been as early as 430 BCE in Athens, but a study published in the journal Cell in 2015 suggests that it began long before that –about 5,353 years before, actually. But perhaps a more assimilable article that outlines the background is found in a BBC news report, also in 2015: http://www.bbc.com/news/health-34603116
‘Samples taken from the teeth of seven bodies contained traces of the bacterial infection in the Bronze Age. They also showed it had, at the time, been unable to cause the bubonic form of plague or spread through fleas – abilities it evolved later.’ You have to love this kind of information, eh?
‘In its early days, it could cause only septicaemic or pneumonic plague – which is nearly always deadly and would have been passed on by coughing. By analysing the bacterium’s genetic code through history, the researchers estimate it took until 1000 BC for plague to evolve into its more familiar form. One mutation – acquiring the ymt gene – allowed the bacterium to survive inside the hostile environment of a flea’s gut. […]Developing a separate gene, called pla, allowed the infection to penetrate different tissues and cause bubonic plague.’
But all things change, don’t they? Things fall apart; the centre cannot hold, in the unforgettable words of Yeats. And yet why would a pathogen evolve to destroy the very hosts on which it depends? Why burn the hotel…?
I suppose an easy explanation might be that of a game in which each side –host/pathogen- continually attempts to outsmart the other. More virulence in the invader leads to more defensive mechanisms in the invaded –things as overt as quarantine or antibiotics, to the more subtle, but hopefully preventative development of immune resources by vaccination or over the longer term, adaptation of endogenous immune defenses: survival of the fittest.
But for me, the intriguingly unanswered question still remains: why kill your host? Why not coexist as, say, a parasite –or even a commensal- in the gut, or create a chronic condition that might weaken the owners, but not eliminate them? Of course, some pathogens are just evolutionary dead-ends – fireworks that illuminate the sky briefly and then disappear as suddenly as they appeared, or maybe finally settle into a desk-job and plod along just under the radar. But I suppose even germs want some time on the pedestal, though. Nothing ventured, nothing gained… Ecological opportunities beg for exploitation –leave a window unlocked, and something will find it.
Of course there are other ways of making a living: attack and retreat to fight again… While not strictly analogous, I am reminded of the Champawat tiger of Nepal (and later in the Kumaon district of India) in the late 19th century. She used to attack suddenly and then disappear before anybody could do anything about her. True, she was finally shot, but not before she’d managed to kill almost 450 people in different locations and instilled fear of her return for years. Fear is like that –especially fear of what Donald Rumsfeld (a once upon a time U.S. secretary of Defence, remember?) oxymoronically called the ‘known unknowns’.
The plague has managed a similar trick over the centuries, flaring up in one region, only to hide, then reappear in a totally different region later –often much later. ‘The most recent plague epidemics have been reported in India during the first half of the 20th century, and in Vietnam during wartime in the 1960s and 1970s. Plague is now commonly found in sub-Saharan Africa and Madagascar, areas which now account for over 95% of reported cases (Stenseth, 2008)’ [https://www.cdc.gov/plague/history/index.html]
But, even those of us living in North America are not entirely safe -remember that Hong Kong plague that Yersin was studying in the 1890ies? A ship from there arrived in San Francisco in the summer of 1899 with plague found among some stowaways, two of whom escaped and drowned in the Bay. An epidemic of plague hit San Francisco nine months later. Whether it was from them or from rats that swam ashore, is not known, but the disease has been with us ever since.
http://www.livescience.com/51792-plague-united-states.html ‘Plague cases occur sporadically in the United States — between 1970 and 2012, an average of seven plague cases occurred yearly […] But plague cases don’t show up everywhere. Rather, most occur in rural areas in western states […] the CDC says. One reason why cases of plague are restricted to the West is that the rodent populations there carry the disease […] “Prairie dogs are one of the major rodent species that serves as a reservoir for plague, and they tend to be west of the 100th meridian” in the United States. For this reason, this line of longitude is sometimes referred to as the “plague line”.’
What, will the line stretch out to th’ crack of doom? asks Macbeth. I suspect that he would have found it fascinating that any of us would think we might be immune from history. And yet, despite all its bad press and the terrifying epithet of ‘Black Death’, plague cases in North America are rare. They can occur when people visit rural areas, says, Dr. Adalja, an infectious disease specialist at the University of Pittsburgh’s Center for Health Security, although ‘people are more likely to be infected with tick-borne illnesses such as Lyme disease, than plague.’
Uhmm, I’d be careful with squirrels in California, though…